A high speed reproduction operation of a video tape recorder (hereinafter referred to as "VTR") using a field memory will be described with respect to a case where it is conducted at a speed of even number times the usual speed such as four times the usual speed. Generally, an odd number times speed is used for high speed reproduction so as to utilize the property that the noisebars are locked due to the fact that the position of the noise of any field is the same as those of the other fields. On the contrary, when signal reproduction is conducted at an even number times speed the position of the noise and the position of the signal are replaced by each other at every field, and if this property is used in using a memory, it might be possible to reduce by half the number of noisebars.
FIG. 1 shows a high speed reproduction system of a prior art VTR. In FIG. 1, the reference numeral 1 designates a video tape onto which a signal is recorded, and the signals reproduced from the video tape 1 by the video heads 2a and 2b are input to the preamplifier 3, and thereafter, the reproduced signals are sent to the video signal processing circuit 4. The reference numeral 5 designates an envelope detector for detecting the envelope of the reproduced signal from the output of the preamplifier 3. The output of the detector 5 is input to the comparator 6 which compares the same with a predetermined level, and the output thereof is sent to memory control circuit 7 which generates timing and address signals for the writing in of the output signal which is output from the video signal processing circuit 4 to the field memory 8. The synchronizing signal is sent to the memory control circuit 7 from the video signal processing circuit 4.
The field memory 8 is a multiport memory (not shown), having a random output port and a serial output port as output ports, and if the serial port is used the writing in and the reading out from the memory can be conducted asynchronously. The operations are conducted asynchronously such that the reproduced signal from the video signal processing circuit 4 is written in the field memory 8 and the content of the field memory 8 is read out with the use of the serial port.
The reference numeral 9 designates a control head, and on the basis of the output of the control head 9 the servo circuit 10 conducts the running control of the tape in each operation mode by controlling the capstan motor 11 and the reel motor 12.
The device operates as follows.
It is supposed that the video tape 1 is conducting a high speed reproduction at a speed four times of the usual speed in the reverse direction. FIGS. 2 and 3 are diagrams for exemplifying the operation at this time. In FIG. 2, the reference numeral 50 designates a video track, the reference characters A and B represent the ozimuth recording, and the video head 2a has the same ozimuth as A and the video head 2b has the same ozimuth as B, respectively.
When the video head 2a traces the locus of the dotted lines (d) in the drawing, the reproduced signal, that is, the output of the preamplifier 3 is as shown in FIG. 3(a) from the relationship in the ozimuth recording. Similarly as above, when the video head 2b traces the locus of the dotted lines (e) in the drawing, the output of FIG. 3(b) is obtained. When the contents of the two fields of FIGS. 3(a) and (b) are mutually interpolated on the time axis, a video image of one field shown as K in FIG. 3(c) is obtained, and this is stored in the field memory 8 and displayed on a monitor. Herein, the envelope waveforms represented in FIG. 3(a), (b), and (c) show only the upper portions of the alternate waveform which has symmetrical configurations at both upper and lower portions.
Such one field content with almost no noisebar is obtained in a case where the width of the video track is approximately equal to or larger than the width of the magnetic head with no guardband. Actually, in a diffused machine having a head construction which co-uses the standard mode and the triple speed mode in a VHS system the specimen of the head is mainly directed to a triple speed mode. When the standard mode recording is conducted in a device having such a specimen a portion of one half to two thirds of the video track 50 becomes a guard band, and the video track becomes as 51 shown in FIG. 4. The outputs of the preamplifier 3 obtained by tracing the loci of the dotted lines f and g by the video heads 2a and 2b on the video track 51, respectively, become as shown in FIG. 5(a) and (b), respectively, and one field video image obtained when these are mutually interpolated on a time axis becomes as shown in FIG. 5(c).
By the way, when the video image interpolated on the time axis shown in FIG. 3 and FIG. 5(c) is written into the field memory 8, the comparison voltage of the comparator 6 must be changed. That is, in FIG. 5(c), if the comparison voltage of the comparator 6 is made "0" level almost all of the envelope is written in into the field memory 8. Herein, although the S/N ratio is low thereat and it appears as a noise region having a noise width of some degree at the portion designated by l because it is almost impossible to obtain a signal information, the interpolated video image information continues smoothly.
However, when "0" voltage is applied to the comparator 6 similarly as above in the waveform K of FIG. 3(c), the portion designated by J is seen with blur because the content thereof is rewritten at every field, and noises appear at the portions designated by m and n at the interval both ends of the J. That is, two noisebars which have noise widths of some degree appear at portions where only a noisebar having quite a small width (approximately a width of one horizontal period) originally appears.
Next, a video tape into which the recording is conducted by a VTR having different video head widths to have a special reproduction function such as a high speed reproduction, a slow reproduction, or a still picture reproduction as its major function will be illustrated in the following.
Suppose that the video tape 1 is reproduced at a high speed of four times the normal speed in a reverse direction. FIGS. 6 and 7 are diagrams for exemplifying the operations then. In FIG. 6, the reference numeral 53 designates a video track recorded on a magnetic tape in a case where the track width of the adjacent tracks A and B are different, that is, the track width of the track A is larger than the track width of the track B. Furthermore, the reference characters A and B designate that ozimuth recordings are conducted, and the video head 2a has the same ozimuth as that of A, and the video head 2b has the same ozimuth as that of B.
When the video head 2a traces the locus of the dotted line H in the drawing, the output of the envelope detector 5 of the reproduced signal becomes as shown in FIG. 7(a) from the relationship on the ozimuth recording. Similarly as above, when the video head 2b traces the locus shown by the dotted line I, the output of FIG. 7(b) is obtained. When the contents of the two fields shown in FIG. 7(a) and (b) are interpolated with each other on the time axis, one field video image shown in FIG. 7(c) is obtained, and this is stored in the field memory 8 and displayed on a monitor. The envelope waveforms shown in FIG. 7(a), (b), and (c) show only the upper portions of the alternate waveforms which have configurations symmetrical at both upper and lower sides.
By the way, when the video image interpolated on the time axis shown in FIG. 7(c) is written into the field memory 8, the comparison voltage of the comparator 6 should be established at a predetermined value, but when the track widths of A and B are different from each other as shown in FIG. 6, there arises an inconvenience if the value of the comparison voltage is established at a constant value.
That is, when the comparison voltage of the comparator 6 is established at a constant value S.sub.1, the portions of N.sub.2 and N.sub.4 are smoothly interpolated to result in a continued screen image, but at the portions of N.sub.1, N.sub.3, N.sub.5 there are no reproduced signals, thereby producing noisebars on the screen. Furthermore, when the comparison voltage is established at S.sub.2, the portions of N.sub.1, N.sub.3, N.sub.5 smoothly continue, but the content is written in twice at every field at the portions of N.sub.2, N.sub.4. These portions are seen with blur on a screen, and noises appear at both ends thereof.
In any way, it is not possible to obtain a high quality video image with a smoothly continuing interpolated image.
As described above, in a device which conducts a high speed reproduction at a speed an even number of times the usual speed with the use of a field memory and conducts the interpolating of the content of each field to obtain an information signal, it is not possible to reduce the number of noisebars and narrow the widths of the noisebars in a case where the comparison voltage of the comparator for comparing the reproduced signal is not changed depending on the video track width on a tape thereby to adjust the writing in range of the memory.